sane.ds: Fix ICAP_BITDEPTH semantics, allow setting it.
Twain ICAP_BITDEPTH is defined in bits per pixel, whereas in sane
depth is defined per color channel. Fix it.
Also allows setting ICAP_BITDEPTH for those sane backends that allow it
sane.ds: Refuse native transfer mode with depth 16 bit.
Native transfer mode on windows can only work with 8 bit
per color component as biBitCount values of 48 for
16 bit per color component are no valid values for a DIB